10.5 Incompatibilities of GCC

There are several noteworthy incompatibilities between GNU C and K&R
(non-ISO) versions of C. The `-traditional' option
eliminates many of these incompatibilities, but not all, by
telling GCC to behave like a K&R C compiler.

GCC normally makes string constants read-only. If several
identical-looking string constants are used, GCC stores only one
copy of the string.

One consequence is that you cannot call mktemp with a string
constant argument. The function mktemp always alters the
string its argument points to.

Another consequence is that sscanf does not work on some systems
when passed a string constant as its format control string or input.
This is because sscanf incorrectly tries to write into the string
constant. Likewise fscanf and scanf.

The best solution to these problems is to change the program to use
char-array variables with initialization strings for these
purposes instead of string constants. But if this is not possible,
you can use the `-fwritable-strings' flag, which directs GCC
to handle string constants the same way most C compilers do.
`-traditional' also has this effect, among others.

-2147483648 is positive.

This is because 2147483648 cannot fit in the type int, so
(following the ISO C rules) its data type is unsigned long int.
Negating this value yields 2147483648 again.

GCC does not substitute macro arguments when they appear inside of
string constants. For example, the following macro in GCC

#define foo(a) "a"

will produce output "a" regardless of what the argument a is.

The `-traditional' option directs GCC to handle such cases
(among others) in the old-fashioned (non-ISO) fashion.

When you use setjmp and longjmp, the only automatic
variables guaranteed to remain valid are those declared
volatile. This is a consequence of automatic register
allocation. Consider this function:

Here a may or may not be restored to its first value when the
longjmp occurs. If a is allocated in a register, then
its first value is restored; otherwise, it keeps the last value stored
in it.

If you use the `-W' option with the `-O' option, you will
get a warning when GCC thinks such a problem might be possible.

The `-traditional' option directs GCC to put variables in
the stack by default, rather than in registers, in functions that
call setjmp. This results in the behavior found in
traditional C compilers.

Programs that use preprocessing directives in the middle of macro
arguments do not work with GCC. For example, a program like this
will not work:

foobar (
#define luser
hack)

ISO C does not permit such a construct. It would make sense to support
it when `-traditional' is used, but it is too much work to
implement.

K&R compilers allow comments to cross over an inclusion boundary
(i.e. started in an include file and ended in the including file). I think
this would be quite ugly and can't imagine it could be needed.

Declarations of external variables and functions within a block apply
only to the block containing the declaration. In other words, they
have the same scope as any other declaration in the same place.

In some other C compilers, a extern declaration affects all the
rest of the file even if it happens within a block.

The `-traditional' option directs GCC to treat all extern
declarations as global, like traditional compilers.

In traditional C, you can combine long, etc., with a typedef name,
as shown here:

typedef int foo;
typedef long foo bar;

In ISO C, this is not allowed: long and other type modifiers
require an explicit int. Because this criterion is expressed
by Bison grammar rules rather than C code, the `-traditional'
flag cannot alter it.

PCC allows typedef names to be used as function parameters. The
difficulty described immediately above applies here too.

When in `-traditional' mode, GCC allows the following erroneous
pair of declarations to appear together in a given scope:

typedef int foo;
typedef foo foo;

GCC treats all characters of identifiers as significant, even when in
`-traditional' mode. According to K&R-1 (2.2), "No more than the
first eight characters are significant, although more may be used.".
Also according to K&R-1 (2.2), "An identifier is a sequence of letters
and digits; the first character must be a letter. The underscore _
counts as a letter.", but GCC also allows dollar signs in identifiers.

PCC allows whitespace in the middle of compound assignment operators
such as `+='. GCC, following the ISO standard, does not
allow this. The difficulty described immediately above applies here
too.

GCC complains about unterminated character constants inside of
preprocessing conditionals that fail. Some programs have English
comments enclosed in conditionals that are guaranteed to fail; if these
comments contain apostrophes, GCC will probably report an error. For
example, this code would produce an error:

#if 0
You can't expect this to work.
#endif

The best solution to such a problem is to put the text into an actual
C comment delimited by `/*...*/'. However,
`-traditional' suppresses these error messages.

Many user programs contain the declaration `long time ();'. In the
past, the system header files on many systems did not actually declare
time, so it did not matter what type your program declared it to
return. But in systems with ISO C headers, time is declared to
return time_t, and if that is not the same as long, then
`long time ();' is erroneous.

The solution is to change your program to use appropriate system headers
(<time.h> on systems with ISO C headers) and not to declare
time if the system header files declare it, or failing that to
use time_t as the return type of time.

When compiling functions that return float, PCC converts it to
a double. GCC actually returns a float. If you are concerned
with PCC compatibility, you should declare your functions to return
double; you might as well say what you mean.

When compiling functions that return structures or unions, GCC
output code normally uses a method different from that used on most
versions of Unix. As a result, code compiled with GCC cannot call
a structure-returning function compiled with PCC, and vice versa.

The method used by GCC is as follows: a structure or union which is
1, 2, 4 or 8 bytes long is returned like a scalar. A structure or union
with any other size is stored into an address supplied by the caller
(usually in a special, fixed register, but on some machines it is passed
on the stack). The machine-description macros STRUCT_VALUE and
STRUCT_INCOMING_VALUE tell GCC where to pass this address.

By contrast, PCC on most target machines returns structures and unions
of any size by copying the data into an area of static storage, and then
returning the address of that storage as if it were a pointer value.
The caller must copy the data from that memory area to the place where
the value is wanted. GCC does not use this method because it is
slower and nonreentrant.

On some newer machines, PCC uses a reentrant convention for all
structure and union returning. GCC on most of these machines uses a
compatible convention when returning structures and unions in memory,
but still returns small structures and unions in registers.

You can tell GCC to use a compatible convention for all structure and
union returning with the option `-fpcc-struct-return'.

GCC complains about program fragments such as `0x74ae-0x4000'
which appear to be two hexadecimal constants separated by the minus
operator. Actually, this string is a single preprocessing token.
Each such token must correspond to one token in C. Since this does not,
GCC prints an error message. Although it may appear obvious that what
is meant is an operator and two values, the ISO C standard specifically
requires that this be treated as erroneous.

A preprocessing token is a preprocessing number if it
begins with a digit and is followed by letters, underscores, digits,
periods and `e+', `e-', `E+', `E-', `p+',
`p-', `P+', or `P-' character sequences. (In strict C89
mode, the sequences `p+', `p-', `P+' and `P-' cannot
appear in preprocessing numbers.)

To make the above program fragment valid, place whitespace in front of
the minus sign. This whitespace will end the preprocessing number.